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A new exhibition tells the haunting stories of the foreign children torn from their families and savagely abused in order to turn them into ‘perfect Aryans’.

The Nazis stole Halina Bukowiecka in 1941 when she was just seven. After her mother died of pneumonia three years earlier and her father had gone to fight for the Resistance, Halina had lived with her grandmother in the German-occupied Polish city of Lodz. Her grandmother's application for child benefits from the city's Youth Office was to have disastrous consequences - it meant the authorities now had Halina on their radar.

One day, Halina and her grandmother were ordered to attend the Youth Office for a 'health check'. There was no reason for suspicion, at least not until the doctors started taking measurements of Halina's face and head, and noted her blonde hair.

Halina was allowed home but was ordered to return a few days later, when her grandmother was told that Halina was being 'temporarily' transferred to an orphanage.

No reason was given and her grandmother was told she had no choice but to obey.

The 'health check' was, in fact, a 'racial examination' and Halina had been deemed 'racially suitable' to be taken from her native Poland to be 'Germanised'. What followed were years of hell, from which she would never psychologically recover. At the orphanage she was forced to speak only German. If she or any of her fellow abductees spoke Polish, they were starved. Worse, the distraught little girl was given a new, Germanic name - Helene Buchenauer.

A few months later, Halina was transferred without her grandmother's knowledge to the Reich School for Ethnic Germans in Achern, near Stuttgart in Germany, some 700 miles west of Lodz. The train journey was almost as bad as those endured by the poor souls sent to such camps as Auschwitz.

Halina and eight of her fellow 'pupils' received no food for the trip, which lasted several days, and it was only thanks to the benevolence of a teacher on board, who shared what little food she had, that the girls survived. At stations en route, the girls begged platform staff for water.

Conditions at the school were a little better. Any pupil who tried to write home was beaten so severely that Halina can still remember their screams. Children were forced to stand at morning roll-call for so long that some fainted. They would be revived by having buckets of iced water thrown over them and were then made to continue standing in their soaking, freezing clothes.

Beatings were commonplace, and to make matters worse for an already devastated Halina, she was endlessly teased for being Polish, and therefore 'inferior' to the school's German-born children.

One day, a very senior Nazi appeared. Halina recognised him straightaway, with his round glasses and small moustache. It was Heinrich Himmler, head of the dreaded SS and, unbeknown to Halina, the man ultimately responsible for her kidnapping and the decades of agony that ensued.

So what was Himmler doing at the school, and why was Halina abducted? The answer lies in a deeply moving exhibition - Stolen Children, Forgotten Victims - that has just opened in Cologne, at the site of the city's former Gestapo headquarters.

The exhibition reveals how the Nazis kidnapped 300,000 children from the territories they invaded, and brought them back to Germany to be raised as racially 'perfect' Aryan children.

This twisted programme of mass child abduction is today almost forgotten, buried by the greater Nazi evil of genocide. Even before the war, Himmler boasted of his idea of plundering other countries for Aryan 'stock' living among supposedly genetically 'inferior' populations.

'Obviously in such a mixture of peoples, there will always be some racially good types,' Himmler observed of other countries' populations in 1938. 'I think that it is our duty to take their children with us, to remove them from their environment, if necessary by robbing, or stealing them.

'Either we win over any good blood that we can use for ourselves and give it a place in our people, or we destroy that blood. I really have the intention to pick up Germanic blood all over the world, to rob and steal, where I can.'

Himmler's chilling plan was put into operation as soon as the Nazis started invading other countries, his pseudoscience wrecking the lives of hundreds of thousands of children and their families.

Among them was Gertruda Niewiadomska, who was born in January 1933. She lived in Poznan in Poland with her foster parents, but was sometimes visited by her mother. One day, in autumn 1940, two men and a woman from the SS came to collect her, although a quick-thinking neighbour managed to hide the girl.

True to their barbaric form, the SS threatened to shoot Gertruda's foster mother unless she gave up the child, which she did the following day.

By that evening, Gertruda was placed with an adoptive German mother, who harshly ordered her to do as she was told. 'I have only been lent to you,' Gertruda snapped back, 'and I won't do it!'

Gertruda's new 'mother' was unimpressed and only too readily gave her up; the next day she was sent to a camp, from which she was deported in a cattle truck to an 'assimilation home' in what is today Kalisz in Poland, about 150 miles from Warsaw.

There, Gertruda was subjected to more racial tests and given the Germanic name of Gertrud Niewermann. The home was very strict, and the principal delighted in beating his charges with his boots. 'We lived in constant fear,' Gertruda would recall.

In 1942, she was adopted by a German family, whom she remembers as being strict and unkind: 'They wanted to turn me into a proper German who was very well disciplined.'

Gertruda's experiences were similar to those of a fellow Pole, a boy called Janusz Bukorzycki, who was born in May 1933 in Lodz. His mother had abandoned him as a baby, leaving him outside a church on a pillow with his name embroidered on it, and he was brought up in an orphanage until he was adopted at the age of three.

In 1943, the Gestapo snatched Janusz to be examined to see if he was suitable for 'Germanisation'. The distance between his eyes was measured, along with the size of his nose and the shape of his skull. Unfortunately, he 'passed' the Aryan test and was abducted from his foster parents and sent to a transit camp.

An SS officer there told him he was now 'Johann Buchner' and he was being sent to the 'assimilation home' at Kalisz.

Janusz was beaten every time he used a Polish word and sometimes was deprived of food for three days. He was also often incarcerated without food for up to 48 hours in a cold basement.

Janusz and his fellow pupils spoke Polish at night only, but were once overheard and forced to march four miles in the middle of a freezing night wearing just their shorts.

In some cases, children were kidnapped by the Nazis during reprisals against the occupied populace. One of the most notorious acts of Nazi savagery came after the assassination of Reinhard Heydrich, the Nazi controller of Bohemia and Moravia, by British trained Czech freedom fighters on May 27, 1942.

A month later, as part of the Nazis' prolonged reprisals, the village of Lidice was destroyed and some 200 men and boys over the age of 15 were lined up in batches of ten and shot dead.

The women and children were sent to concentration camps, but not before some of Himmler's men evaluated the children for racial suitability for 'Germanisation'.

Among seven children whose Aryan looks saved them from the camps were Vaclav and Anna Hanf, who were separated from their mother and deported to Lodz. There, they were made to sleep on a cold stone floor in an orphanage for almost a year and, as with so many other children, beaten for using their mother tongue.

After an unhappy spell with a family called Strauss, the brother and sister were separated, and Vaclav was sent to a Lebensborn home in Austria. The Lebensborn movement (literally 'font of life') was key to Himmler's abduction programme.

The Lebensborn movement is normally associated with an SS-funded initiative to breed racially 'pure' Aryans. In territories the Nazis conquered, German troops and SS men were encouraged to procreate with Nordic women in order to further Himmler's dream of an Aryan master race.

Many of the children born from these unions were raised in special Lebensborn homes, which were also used to house abductees.

Conditions were predictably harsh. Once, Vaclav ignored an order and was locked up for ten days in a basement. Another time, he had a knee broken for refusing to learn how to use a rifle. Vaclav was then moved to a boarding school, where he was made to watch something that would haunt him for ever.

'One day, we were made to gather in a courtyard and the SS men led in a boy who was about 12 and was, I believe, a Pole,' Vaclav recalls, adding that the child's 'crime' was unknown. 'The boy had his head put on a pad and an SS man with a long butcher's axe chopped off his head. We were all threatened with similar treatment if we committed any infringements.'

With so many thousands of children kidnapped, there are so many more horrific stories that could be told, and the exhibition could potentially take up a building the size of nearby Cologne cathedral.

So what happened to these children?

Some, such as the Hanf siblings and Janusz Bukorzycki, made it back to their homelands.

Others such as Gertruda Niewiadomska would, devastatingly, not discover their true ancestry until late in life.

Among those still suffering is Hermann Ludeking, who was adopted from Lodz, where he was born Roman Roszatowski.

'All my life I've been looking for my parents,' he says, 'and they have never been found. I ask myself: "Who am I? Where do I come from? Who are my parents?" I know I will never find an answer. To this day, I am plagued by these thoughts.'

What particularly riles the likes of Hermann is the lack of compensation given to him and his fellow stolen children.

'I feel no differently from the Jews or the forced labourers who were deported,' he says. 'We stolen children have been let down by the German government.'

Herr Ludeking has a point. Whereas the German authorities have paid Israel and the World Jewish Congress some £6 billion in today's money, and around £3 billion to former forced labourers, those such as Herr Ludeking have not received a penny.

For Halina Bukowiecka, returning to her homeland after the war was a mixed blessing.

Her grandfather regarded her as a 'German child' and largely rejected her. At school, her Russian teacher saw her as a de facto Nazi and made Halina feel responsible for German atrocities.

When the exhibition's curator, Christoph Schwarz, asked if her psychological wounds had healed after some 70 years, Halina broke down and wept.

'Her tears speak for themselves,' Herr Schwarz observes.

Ultimately, Himmler's plan did not just steal children and their childhoods, but it stole entire lives.

Chilling: Heinrich Himmler checks children in 1941 for their 'Germanic' features